Frequency stabilizing arrangement



y' 1940- CARL-ERIK GRANQVIST 2,201,770

FREQUENCY STABILIZING ARRANGEMENT Filed Oct. 8, 1937 DETECTOR 8 .2

PLATE vourls INVENTOR GAR ERIK GRANQV 1'' 6 4 'HGIAVIAEQ AQNQEBEBM 1Patented May 21, 1940 UNITED STATES PATENT OFFICE Carl-Erik Granqvist,Stockholm, Sweden, assignor to Hazeltine Corporation,

Delaware Application October 8,

a corporation of 1937, Serial No. 167,911

In Sweden May 15, 1937 6 Claims.

This invention relates to a frequency-stabilizing arrangement and, whileit is of general application, it is particularly suitable forstabilizing the frequency of an oscillator, for example, the localoscillator of a superheterodyne receiver.

In a superheterodyne receiver comprising a multi-grid vacuum tube whichfunctions as an oscillator-modulator andv which is subject to automaticvolume control, a change takes place in the slope of the grid-voltageplate current characteristic, or transconductance, of the tube when anautomatic amplification control potential is applied to an electrode ofthe tube. The frequency-determining circuit of such anoscillator-modulator includes the transformed interelectrode impedancesof the tube. Since these impedances are altered by variation of theautomatic amplification control potential, it is evident that theresonant frequency of the frequency-determining circuit iscorrespondingly altered. In superheterodyne receivers, this results in adisplacement of the intermediate frequency developed by theoscillator-modulator tubefrom the mean resonant frequency of theintermediate-frequency selector circuits. The detuning resulting fromthe variation in automatic amplification control bias to theoscillatormodulator tube effects a distortion similar to that normallydue to manual detuning. Such detuning distortion has substantiallylimited the range of automatic amplification control.

It is an object of the invention to provide a stabilized oscillator, thefrequency of which is not substantially altered by variations of thetransconductance of the tube.

It is a further object of the invention to provide a stabilizedoscillator-modulator for superhet'erod'yne receivers such that theintermediate frequency is not substantially altered by variations ofautomatic amplification control potential applied to theoscillator-modulator tube.

In accordance with one embodiment of the invention, a high-frequencyoscillator comprises a vacuum tube having input and output electrodes, afrequency-determining circuit coupled to the input electrode, and aninductive energy-transfer coupling between the frequency-determiningcircuit and an output electrode, whereby the oscillator is caused tooscillate at the frequency of the frequency-determining circuit. Meansare provided for varying the transconductance of the tube, therebytending to vary the output frequency of the oscillator, While there arealso provided separate means comprising an energyof the inductiveenergy-transfer coupling between the frequency-determining circuit andan output electrode and including reactance and resistance of largeimpedance relative to the reactance, for coupling into thefrequency-determining circuit a reactive componentvariable with thetransconductance of the tube to compensate the abovementioned tendency.

For a better understanding of the invention, together with other andfurther objects thereof, reference is had to the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

Fig. 1 is a circuit diagram of a vacuum-tube repeater havingcapacitively coupled grid and anode and useful in deriving thefundamental relationships involved in the invention; Fig. 2. is acircuit diagram of an oscillator with inductive feed-back coupling alsouseful in deriving fundamental circuit relations; Fig. 3 is a circuitdiagram of an oscillator combining the features of Fig. l and Fig.2;Fig. 4 is a simplified equivalent circuit diagram of the circuit of Fig.3; Fig. 5 illustrates an embodiment of the invention in an oscillatorcomprising a heptode vacuum tube; Fig. 6 is a graph illustrating certaincharacteristics of the circuit of Fig. 5; and Fig. 7 illustrates anembodiment of the invention in the oscillator-modulator of asuperheterodyne receiver.

It is well known that it is possible to cause the input circuit of avacuum tube to simulate a variable condenser or a variable inductance.This effect of the input circuit of a Vacuum tube has been called theMeissner effect or Miller efiect. The capacitive or inductive reactanceof such a device may be varied by varying the slope of the grid-voltageplate current characteristic, or transconductance, of the vacuum tube asby varying the bias of the control grid of the tube. Fig. 1 shows anexample of such arrangement. The input terminals of vacuum tube 3 arerepresented by numerals l and 2. The control grid is coupled through thecondenser 4 to the terminal I and the cathode is connected to terminal2. The tube 3 may "be, as indicated, of the pentode type. A resistance 5is connected in the anode circuit and the anode is capacitively coupledto the grid by means of a condenser 6 in series with condenser 1. Thecontrol grid is connected to the grounded cathode of the tube through agrid leak I and a source of bias potential, such as the battery 8. Theimpedance Z between terminals l and 2 is then given by the followingequation:

Equation 2 shows that the effective capacity included in the impedance Zmay be varied by a variation of the transconductance gm of the tube asby a variation of the bias from the source 8. Thus, the effect of thearrangement shown in Fig. 1 is that a reactive component is fed back tothe grid. circuit through the reactive coupling between the anodecircuit and the grid circuit of the tube with the result that the inputcircuit of the tube simulates a condenser having a capacitance variablewith the bias potential supplied to the grid by source 8.

By providing a coupling between the anode circuit and the grid circuithaving an opposite phase relation to the arrangement of Fig. 1 it ispossible to cause the grid circuit of the tube to simulate an inductancevariable with bias of the tube. Fig. 2 illustrates such an arrangementin which elements similar to those in Fig. l are given the samereference numerals. An inductive coupling between the anode circuit andthe grid circuit is effected by connecting in the plate circuit aninductance 9 inductively coupled to a winding H] in the grid circuit.The mutual inductance between inductances 9 and I 0 produces an effectcorresponding, but opposite, to the effect of the coupling condenser 6of Fig. 1. The impedance between points i and 2, therefore, has aresistive component as well as an inductive component. With the additionof a variable condenser l2 and a padding condenser H in series withinductance I0, forming a frequencydetermining circuit, the arrangementof Fig. 2 functions as an oscillator. The series impedance Z1 ofinductance element l0 and condenser H is given by the equation:

in which:

r1=resistance of inductance element l0, represented at H1 in thedrawing. 1J=Heaviside operator.

L1=inductance of coil I0.

C1=capacitance of condenser H.

If the efiect of the mutual inductance transformed through tube 3 istaken into consideration, the elfective impedance Z2 of circuit H], H isas follows:

V=voltage between points I and. 2.

I=current through inductance H] and condenser I I.

gm=transconductance of the tube 3.

M=mutual inductance between inductance elements 9 and I0.

It should be noted that the mutual inductance M is, in this case, anegative quantity. By varying the transconductance of the tube as byvariation of any of the electrode operating potentials, the impedance ofcircuit H), II is correspondingly changed. From this it follows that theresonant frequency of the frequency-determining circuit of theoscillator is also varied. The present invention is elfectivesubstantially to eliminate such frequency variations.

An embodiment of the invention is shown in Fig. 3, which is acombination of the circuit arrangement shown in Figs. 1 and 2. Theanalysis of the arrangement of Fig. 3 is best carried out by consideringseparately the eifect of the feedback through the mutual inductancebetween inductances 9 and 0 and that through condenser 6. Accordingly,the circuit will be considered without regard to the mutual inductancebetween inductances 9 and ID. The equivalent schematic circuit for suchan arrangement is shown in Fig. l. The impedance Z3 between theterminals l and 2 of Fig. 4 is given by the equation:

Z )1: l 1 1 l x 3 :DCI PC A All symbols have the same significance as inthe preceding equations. If the capacitance C1 is made relatively large,as is usually the case, the quadratic factor in the right member in thenumerator becomes infinitesimal and, as a first approximation, equation(5) may be written:

in the second denominator in Equation 6 is small relative to the termR1001 and the first two terms in the second denominator in Equation 6may be disregarded.

Equation 6 then reduces to the following:

Zg Z1 1 I 7 1/ A comparison of Equations 4 and 7 indicates thatrepresenting the inductive effect given by Equation 4 is the samemagnitude as the factor representing the capacitive effect given byEquation '7 if 21rfM is chosen to be equal to 1 will- It should benoted, however, that Equation 8 is not exact, inasmuch as certainapproximations have been made in the transition from Equation toEquation 7, whereas Equation 4 has not been approximated. Thus, acomplete compensation of the inductive coupling effect by means of thecapacitive coupling effect is not procured. It has been found, however,both by calculation and by experiments, that the inaccuracies due to theapproximations are so small as to be of no practical significance. Inorder to obtain the greatest possible. accuracy in compensation throughan arrangement according to the invention, the terms appearing inEquation 5 should be given appropriate values to the extent to which oneis free to choose their dimensions. If Equation 5 is transformed in aknown way from vectorial to algebraic structure, and if Z3 isdifferentiated partially with respect to C and with respect to R, thecondition for the greatest variation of Z3 with respect to C and R isfound to be RwC'=l; that is or the impedance of C is equal to theimpedance of R at the frequency of greatest variation of Z3. If theoscillator is designed to operate at a constant frequency, thiscondition can evidently be satisfied. If, on the other hand, theoscillator is tunable over a range of frequencies, the circuit should bedesigned so thatthe above-indicated condition is satisfied at the meanfrequency of the range.

Another embodiment of the invention is illustrated in Fig. 5. Thiscircuit differs from that of Fig. 3 only in that inductance 9 is notconnected in the main plate circuit, but in an auxiliary anode circuitcomprising the auxiliary anode I3. A cathode-biasing resistor l4 shuntedby a by-pass condenser [5- provides a bias source for tube 3 equivalentto source 8 of Fig. 1. The operation of the arrangement is in allrespects similar to that described above with reference to Fig. 3.

The relation between frequency deviation and anode voltage of anoscillator embodying the circuit of Fig. 5 is shown in Fig. 6. Twodifferent scales have been used as ordinates, that is, curve at havinglarger frequency units than that for curve b to enable both curves to beshown on the same diagram and at the same time to be kept withinreasonable limits. The abscissae for both curves indicate the platevoltage of the tube. Curve a shows the frequency deviation of anoscillator not incorporating the feed-back circuit of the invention,while curve b shows the frequency deviation of such an oscillatorembodying the invention and having a smaller frequency scale. It is seenthat, in the latter case, the frequency deviation is practirallyconstant with variation of the anode voltage. While Fig. 6 shows thedeviation in frequency vs. anode voltage, corresponding characteristicsfor the circuit of Fig. 3 would preferably be shown as deviation infrequency vs. grid.

tional portions are indicated schematically since, per se, they form nopart of the invention. This receiver comprises, in cascade, an antennacircuit 20, 2|, a tunable radio-frequency selector 22, 23,afrequency-changer or oscillator-modulator tube 24, anintermediate-frequency selector system comprising the double-tunedtransformer 42, 43 and 44, 45, an intermediate-frequency amplifier 60, adetector and automatic amplification control supply 6!, anaudio-frequency amplifier 62 of one or more stages, and a loud-speaker63.

The oscillator-modulator 24 comprises an oscillation network 34, 35 and,in order to tune the radio-frequency selector network 22, 23 andoscillation network 34, 35 in unison, the tuning condensers 23 and 35are ganged for uni-control as'shown in the drawing. Neglecting for themoment the parts of the system involving the present invention, thesystem described above includes the features of a conventionalsuperheterodyne receiver; the operation of such receiver being wellunderstood in the art, detailed explanation is deemed unnecessary.Briefly, however, a desired modulated-carrier signal intercepted by theantenna 20 is selectively amplified in radio-frequency selector 22, 23and converted in frequency changer 24 to an intermediate-frequencymodulated-carrier signal. This signal is selectively amplified by theintermediate-frequency selector comprising double-tuned transformer 42,'43 and 44, 45 and amplifier 60, and translated therefrom to thedetector and A. V. C. supply 5|, where the audio frequencies ofmodulation and the automatic amplification control-bias potentials arederived. The audio frequencies of modulation are amplified in theaudio-frequency amplifier 52 and reproduced in loud-speaker 63 in aconventional manner. Automatic amplification control potentials aresupplied from unit 6| to the control grid 25 of oscillator-modulator 24and to one or more tubes of intermediate-frequency amplifier 60. 1

Coming now to the parts of the system involved in the present invention,the cathode 21 of tube 24 is grounded in the usual way through acathode-bias resistor 30 shunted by a by-pass condenser 3|. Theoscillator control grid 28 is connected to the cathode through grid leak32 and coupled to the frequency-determining circuit 34, 35 throughcondenser 33. The oscillator anode 29 is coupled to a source of positivevoltage, indicated as +B, through anode resistance 31 and to thefrequency-determining circuit through condenser 38 and inductance 39,inductively coupled to inductance 34. Condenser H corresponds tocondenser II of Figs. 2P5, inclusive. The main anode 40 of the tube 24is coupled through an anode resistance 4! to the primary Winding 42 ofthe tuned intermediate-frequency transformer 42, 43 and 44, 45. Thetuned secondary circuit 44, 45 is connected to the input circuit ofintermediate-frequency amplifier 60 in a conventional manner.

When there is being received a station having signals of widely varyingamplitude due to fading or other causes, an automatic amplificationcontrol bias variable within wide limits is impressed on the outercontrol grid 25 of tube 24 through the above-described arrangement. Thisvariation in the grid bias of tube 24 varies its transconductance and,as was brought out above, tends to cause the oscillator frequency to besubjected to a corresponding variation. To compensate for this variationthe condenser 51 and resistance 4-! are provided according to theinvention. The condenser 5'! couples into the frequencydeterminingcircuit 34, 35 a potential derived from the anode circuit of tube 24 andof proper phase to compensate for the tendency of the frequency of theoscillator to vary with varying transconductance of the tube 24, asdescribed above in connection with Figs. 3-6, inclusive.

It will be understood that the coupling between electrode 29 and thefrequency-determin-- ing circuit 35, 35 comprising the coupling path 38,39, as well as the coupling between anode 40 and thefrequency-determining circuit 34, 35 comprising the coupling path 57,39, are each referred to in this specification as a coupling between thefrequency-determining circuit and an output electrode of the tube. Thereis thus provided a separate means comprising an energytransfer couplinghaving a phase opposite to that of the inductive energy-transfercoupling between the frequency-determining circuit 34, 35 and an outputelectrode of vacuum tube 24. This separate coupling means comprises thereactance of condenser 51 and inductance 39, as well as the resistanceof resistor ii of large impedance relative to the reactance of thecoupling path, for coupling into the frequency-determining circuit areactive component which is variable with transconductance of the tube24 to compensate the tendency of variations in the transconductance ofthe tube to vary the output frequency of the oscillator.

While there have been described what at present are considered to be thepreferred embodiments of the invention, it will be apparent to thoseskilled in the art that various changes and modifications may be madewithout departing from the invention, and it is, therefore, aimed in theappended claims to cover all such changes and modifications as fallwithin the spirit and scope of the invention.

What is claimed is:

l. A high-frequency oscillator comprising a vacuum tube having input andoutput electrodes, a frequency-determining circuit coupled to said inputelectrode, an inductive energy-transfer coupling between saidfrequency-determining circuit and an output electrode, whereby saidoscillater is caused to oscillate at the frequency of saidirequency-determining circuit, means for varying he trans-conductance ofsaid tube thereby tending to vary the output frequency of saidoscillator, and separate means comprising an cnergy-transfer couplinghaving a phase opposite to that of said inductive energy-transfercoupling between said frequency-determining circuit and an outputelectrode and including reactance and resistance of large impedancerelative to said reactance for coupling into said frequency-determiningcircuit a reactive component variable with said transconductance tocompensate said tendency.

2. A high-frequency oscillator comprising a vacuum tube having input andoutput electrodes, a frequency-determining circuit coupled to said inputelectrode, an inductive energy-transfer coupling between saidfrequency-determining circuit and an output electrode, whereby saidoscillater is caused to oscillate at the frequency of saidfrequency-determining circuit, means for varying the trans-conductanceof said tube thereby tending to vary the output frequency of saidoscillator, and means comprising a resistor in an output electrodecircuit of said tube and a condenser coupledbetween said last-mentionedoutput electrode and said frequency-determining circuit for couplinginto said frequency-determining circuit a reactive component variablewith said transconductance to compensate said tendency.

3. A high-frequency oscillator comprising a vacuum tube having input andoutput electrodes, a frequency-determining circuit including a condenserand an inductance in one of its arms and coupled to said inputelectrode, an inductive energy-transfer coupling between saidfrequencydetermining circuit and an output electrode, whereby saidoscillator is caused to oscillate at the frequency of saidfrequency-determining circuit, means for varying the transconductance ofsaid tube thereby tending to vary the output frequency of saidoscillator, and means including a resistor in. an output electrodecircuit of said tube and a condenser coupled between said last-mentioned output electrode and the junction of said inductance and saidfirst-mentioned condenser for coupling into said frequency-determiningcircuit a reactive component variable with said transconductance tocompensate said tendency.

4. A high-frequency oscillator comprising a vacuum tube having input andoutput electrodes, a frequehey-determining circuit coupled to said inputelectrode, an energy-transfer circuit between said frequency-determiningcircuit and an output electrode including an inductance inductivelycoupled to said frequency-determining circuit, whereby said oscillatoris caused to oscillate at the frequency of said frequency-determiningcircuit, means for varying the transconductance of said tube therebytending to vary the output frequency of said oscillator, and meansincluding a resistor in an output electrode circuit of said tube and acondenser in series with said inductance and coupled between saidlast-mentioned output electrode and said frequency-determining circuitfor coupling into said frequency-determining circuit a reactivecomponent variable with said transconductance to compensate saidtendency.

5. A high-frequency oscillator comprising a vacuum tube having input andoutput electrodes. a tunable frequency-determining circuit coupled tosaid input electrode, an inductive energy transfer coupling saidfrequency-determining circuit and an output electrode, whereby saidoscillator is caused to oscillate at the frequency of saidfrequency-determining circuit, means for varying the transconductance ofsaid tube thereby tending to vary the output frequency of saidoscillator, and means including a resistor in an output electrodecircuit of said tube, and a condenser coupled between saidlast-mentioned output electrode and said frequency-determining circuitfor coupling into said frequency-determining circuit a reactivecomponent variable with said transconductance to compensate saidtendency, said condenser having a reactance equal to the mutualinductive reactance between said frequency-determining circuit and saidfirst-mentioned output electrode at the frequency at which maximumcompensation is desired.

6. A high-frequency oscillator comprising a vacuum tube having input andoutput electrodes, a frequency-determining circuit coupled to said inputelectrode, an inductive energy-transfer coupling between saidfrequency-determining circuit and an output electrode, whereby saidoscillator is caused to oscillate at the frequency of saidfrequency-determining circuit, means for varying the transconductance ofsaid tube thereby tending to vary the output frequency of saidoscillator, and means including a resistor in an output electrodecircuit of said tube and a condenser coupled between said last-mentionedoutput electrode and said frequency-determining circuit for couplinginto said frequency-determining' circuit a reactive component variablewith said transconductance to compensate said tendency, said resistorhaving an impedance equal tothat of said condenser at the frequency atwhich maximum compensation is desired.

CARL-ERIK GRANQVIST-

